The present invention relates to electric motors and, more particularly, to linear electric motors.
Electric motors conventionally include a stationary portion, or stator, and a movable portion, or rotor. In electric motors adapted for turning an output shaft, the rotor is concentrically arranged with the stator. Although thee most common rotary motor employs a cylindrical rotor rotating within an annular stator, the reverse arrangement is not unknown.
An electric motor of the linear type has more recently found use in certain applications. A linear motor employs a stator which can be thought of as being similar to the stator of a rotary motor which has been developed, or opened out, into a flattened assembly. Similarly, the rotor of the rotary motor is replaced by a linear counterpart which moves in a line with respect to the stator.
Alternating current versions of linear motors of the synchronous type are disclosed in U.S. Pat. Nos. 3,594,622; 3,699,365 and 3,706,922, among many others. Linear motors of the induction type are disclosed in U.S. Pat. Nos. 3,770,995; 3,824,414 and 3,884,154, among many others.
An even more recent type of linear motor employs direct current using either wound field-generating coils on both the moving and stationary elements or using one wound element and one element which employs one or more permanent magnets. Such a linear motor is disclosed in U.S. patent application Ser. No. 383,351, invented by the present applicant. In the referenced patent application, a stator employs a U-shaped channel to contain the stator field-generating elements. A movable element is guided to move along the axis of the U-shaped channel. An apparatus to be linearly displaced is connected to the moving element.
Linear motors of both the AC and DC types frequently operate in a factor, or other applications, where the environment is far from clean. Dust, dirt and corrosive gases may be present in such applications. Such contaminants, if permitted access to the interior of a linear motor can, at the least, cause a cleaning problem and, in more severe situations, can cause shorting, breakdown or failure of commutation. One solution has been the employment of accordion-pleated flexible jackets sealed about the linear motor to exclude environmental contaminants. Such jackets are typically expensive and their bulk interferes with ready access to the linear motor.
A further problem which must be addressed with al electric motors, and particularly those in which a substantial amount of power must be dissipated, is the maintenance of a reasonable temperature in the elements of the linear motor. In the referenced patent application, the amount of heat generated by the wound stator is reduced by a commutation technique wherein only those coils within the influence of the magnitic field of the moving element are energized at an time. Thus, a region of the stator in which heat may be generated moves along the stator with the moving element. In motors of relatively low power dissipation, convection cooling can be relied on to avoid overheating. At higher power dissipations, some type of forced cooling is required.